Do central or marginal rays bend more?

• A. Central
• B. Marginal
• C. They bend equally
• D. Cannot be determined

Answer: (B)

Refraction depends on how large the angle is between the incoming ray and the surface normal. Central rays hit the surface near the optical axis where the surface is nearly perpendicular to the ray, so their angle of incidence is small and they bend only a little. Marginal rays strike farther from the axis where the surface tilts more relative to the ray, giving a larger angle of incidence and a greater change in direction according to Snell’s law. Because of this, the edge (marginal) rays bend more than the central rays, leading to different focal points for the two sets of rays.